Abstract

Abstract

Nitric oxide reacts with C2H2 at ambient temperature under aerobic conditions with relatively large quantities of O2 present. We used a gas-flow system at 20°C to determine the extent, if any, of reaction of NO with C2H2 under nearly anaerobic conditions with addition of trace (<500 µL L−1) quantities of O2, and to determine whether the process could account for the apparent C2H2 inhibition of NO production in soil. Results showed that reaction occurred only when critical trace quantities of O2 were present. For example, if [NO] was in the range 0.5 to 2.5 µL L−1, which is typical of concentrations in our soil studies, approximately 200 to 300 µL O2 L−1 was required before measurable NO loss was observed. The reaction was first order with respect to NO and variable order, becoming zero order, with respect to O2 when [O2] exceeded ≈400 µL L−1 and maximum NO depletion rates (≈0.01–0.06 µL L−1 s−1) were obtained. The effect of C2H2 on NO production with addition of O2 (≈20–730 µL L−1) in Brookston clay loam (fine-loamy, mixed, mesic Typic Argiaquoll) and Fox sandy loam (fine-loamy over sandy or sandy-skeletal, mixed, mesic Typic Hapludalf) under conditions similar to the gas-phase studies was also investigated. Depletion of NO in the presence of C2H2 vs. [O2] followed the same pattern with similar rates as the gasphase process. A simplified C2H2-catalyzed scheme for the oxidation of NO to NO2 provides a plausible, partial explanation for these results. Since NO depletion requires only trace amounts of O2, the results suggest that in many previous studies involving assays of NO or N2O production based on C2H2 inhibition, significant gas-phase reaction may have occurred leading to serious errors of interpretation.

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